Environment International 139 (2020) 105717

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Environment International

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Typical diffuse loading and degradation pattern differences under the impacts of climate and land-use variations T ⁎ Wei Ouyanga, , Xin Haoa, Mats Tysklindb, Wanxin Yanga, Chunye Lina, Aihua Wanga a State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China b Environmental Chemistry, Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden

ARTICLE INFO ABSTRACT

Handling Editor: Olga-Ioanna Kalantzi Riverine sediment can reconstruct the history of organic pollution loads and can provide reliable temporal Keywords: information for metabolite dynamics in watershed. Sediment core samples were collected from two Sedimentary pattern riverine sections of a cold watershed in the presence land use change under agricultural development, and Diffuse pollution the vertical concentrations of four pesticides (, prometryn, isoprothiolane, and oxadiazon) and two Atrazine degradation atrazine metabolites (deisopropyl-atrazine and deethyl-atrazine) were determined by –mass Pesticide spectrometry. The presence of pesticides and metabolites was detected at different depths (11–17 cm) at 1-cm Watershed modeling intervals along the two sediment cores, and the flux was calculated with a constant rate of supply model based on the observed concentrations and 210Pb isotope radioactivity chronology. By comparing the concentrations and fluxes of pesticides between the two sediment sections, significant differences in accumulation under different land-use patterns were found. Redundancy analysis further indicated that temporal watershed farmland variance was the dominant factor for pesticide loading. The lower concentration of atrazine and the higher concentration of the other pesticides in the estuarine sediment was closely related to the decreasing upland in the upstream area and the increase in paddy fields in the downstream area. The analysis of atrazine and the metabolites indicated that atrazine is more likely degraded to deethyl-atrazine and the metabolites have similar migration processes in the sediments, which can easily migrate downward. Moreover, the ratio of metabolites to atrazine showed that atrazine degradation was intensive during the transport process, but the metabolites efficiency was lower in this area due to the cold temperature. The results provide insights for the management of pesticide pollution control in watersheds and the potential effects of low temperature on the degradation of pesticides.

1. Introduction accumulate in surface water and sediment, which can lead to a lasting potential threat to the aquatic environment (Grung et al., 2015; Kaonga The use of pesticides is of great significance in ensuring food se- et al., 2015, Lapworth et al., 2015). Understanding pesticides’ migra- curity, but with agricultural expansion and the excessive use of pesti- tion, degradation and accumulation in surface water and sediment can cides, the impact of pesticide on ecosystem security has become an offer insights into pesticide pollution management. important issue of global concern (Anderson et al., 2002; Tilman et al., Sediment in the watershed outlet represents the final compartment 2002). Although pesticides are trace agricultural diffuse-source pollu- for pesticide accumulation, which persists for a long time in aquatic tants, the high environmental risk and persistent impact associated with sediments; thus, sediment at the outlet is an effective medium that can pesticides and their related metabolite products cannot be ignored be used to assess the contamination level and identify the sources of (Love et al., 2011; Braun et al., 2019). The transport and degradation contamination of watersheds that have mixed land use (Fairbairn et al., fate of pesticides in agricultural watersheds comprise complex pro- 2015; Bhattacharya et al., 2003). Studies have shown that watershed cesses, which involve hydrolysis, photolysis, adsorption, oxidation, trace organic pollutants are enriched in the sediments, and sediment volatilization, photodegradation, and microbial degradation (Ghattas pollution is more stable over time than are dissolved or suspended et al., 2017; Qu et al., 2017). Most of these pesticides are hydrophobic pollutants; thus the accumulation of pesticides in the sediments can be contaminants; thus, they can be transported from farmland into the used to evaluate the anthropogenic and natural impacts that result from aquatic environment by adsorbing onto soil particles, and they the entire watershed (Ghosh et al., 2016; Chapman et al., 2013). In

⁎ Corresponding author. E-mail address: [email protected] (W. Ouyang). https://doi.org/10.1016/j.envint.2020.105717 Received 30 December 2019; Received in revised form 24 March 2020; Accepted 2 April 2020 Available online 10 April 2020 0160-4120/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). W. Ouyang, et al. Environment International 139 (2020) 105717 addition, because the sediment is a sink for pollutants, the type and selected, both of which had a small slope of 5 degrees and no river concentration of the pesticides in sediment in different riverine sections branch or sand mining. The first section (S1) was located at the wa- can also reflect the spatial distribution of land use and the differences in tershed outlet, which has paddy land in the downstream area, and the crop tillage management (Moran et al., 2017). second section (S2) was located along the upland boundary in the upper In addition to the properties of pesticides, the sedimentary process is stream area. Based on the date and the local hy- largely dependent on soil erosion particle properties which also control drological conditions, the sediment cores were collected in July 2015 the biological characteristics and the hydrological conditions that affect after the application. Sediment cores of 30-cm were collected from each pesticide transport and degradation (Boyer et al., 2018; Chattopadhyay section using a columnar sampler (PVC tube, 7.5 cm in diameter), and and Chattopadhyay, 2015). Field observations and laboratory experi- samples were sliced into 1-cm intervals to collect a total of 60 samples. ments have shown that the sorption and degradation processes of pes- Samples were transferred to polyethylene bags and were freeze-dried, ticides are directly affected by soil organic matter and the soil aerobic slightly crushed, passed through a 0.147-mm sieve and stored in sealed and anaerobic conditions also have impacts (Aparicio et al., 2018; Dang bags (Ouyang et al., 2017b). et al., 2018). In some high-altitude agricultural watersheds of China, The main types of crops being grown in the watershed are soybean, pesticides and their metabolites are detained in the frozen soil during maize and rice. Prometryn, isoprothiolane, and oxadiazon are the main the winter period, and then they are transported to the river and moved pesticides used in rice paddies, (and oxadiazon is also used), and they downstream during the thawing process that occurs in the spring were used on 7 May, 5 May and 6 July, respectively (Fig. 1b). Atrazine (Ouyang et al., 2017a,b). However, little is known about the long-term was introduced in the 1950s and has been widely used for weed control accumulation and metabolic characteristics of pesticides in sediments in the uplands. Atrazine was used in the uplands on 1 June with a in agricultural watersheds with cold temperature conditions. The dosage of 0.79 kg/ha. The freeze-thaw cycle in the agricultural wa- characteristics of pesticides could have different performance in cold tershed provided the opportunity to preliminarily study the transport of watersheds with a high content of soil organic matter. typical pesticides and to characterize the pesticide metabolites under As a stable medium in the watershed, sediment provides a route cold conditions. As an organic pollutant, the degradation process of map for the historical inversion analysis of a typical pollutant under a atrazine and its impact on environmental health are also main concerns changing environment (Mcmurry et al., 2016). With advances in geo- (Tierney et al., 1999). Based on the land use distribution and transport chemical methods, sediment dating has become a more effective distance, atrazine was selected to accomplish this goal. Atrazine has a method to reconstruct the historical pollution pattern; and the activity half-life of more than 200 days in surface water and a moderately hy- 210 210 226 of Pbex (210Pbex = Pbtot − Ra) is commonly used to calculate drophilic characteristic (its water solubility is 33.8 mg/L at 22 °C, sediment age through a the constant rate of supply model pKa = 1.68, Log Kow = 2.5) (Guo et al., 2016). Atrazine can degrade to (Krishnaswamy et al., 1971; Townsend and Seen, 2012). The results of deisopropyl-atrazine (DIA) and deethyl-atrazine (DEA) (Fig. 1b); both radiometric dating can be integrated with other factors to determine the are especially recalcitrant to and have strong leaching temporal patterns of pesticide pollution (Della et al., 2016). Therefore, properties (the water solubility of DIA and DEA is 3200 mg/L and combining the dating results and the concentrations of the pesticides in 670 mg/L, respectively; pKa, DIA = 1.30–1.58, pKa, DEA = 1.30–1.65; the sediment can highlight the impacts to the conversion of farmland on Log Kow, DIA = 1.15, Log Kow, DEA = 1.52) (Vryzas et al., 2012). They the discharge of pesticides in watersheds with developing . also have varying degrees of persistence and toxicity, especially DEA, There have been some innovative studies that have highlighted the which is phytotoxic and has been reported to have a stronger effect on fate of pesticides in the agricultural system and assessed diffuse loading aquatic life (Mauffret et al., 2017). to the aquatic environment (Bhattacharyya et al., 2011; Potter and Coffin, 2017). However, the comparisons of pesticides and their meta- 2.2. Extraction and pesticide analysis bolites in riverine sediments in a watershed with cold temperatures and a high content of soil organic matter and the driving forces for long- To highlight the pollution sedimentary dynamics, the concentra- term changes in pesticides are still rare. Based on field investigations tions of four typical pesticides and two atrazine metabolites were and the integration of other methods, the specific objectives of this analyzed using gas chromatography–mass spectrometry (GC–MS) study are the following: (1) explicitly identify the effects of the spatial (Fig. 1b). The procedure was applied for the simultaneous distribution of land use on the migration and accumulation of pesticides extraction of pesticides from sediment. Briefly, 6 g of dry sediment with the concentration and flux of four typical pesticides in two sections sample was weighed in a 50-mL polypropylene tube. Then, 10 mL of from one watershed, (2) understand the metabolite dynamics of atra- ultra-pure water and 20 mL of acetonitrile were added, and the zine and the sedimentary and transport characteristics of metabolites in mixture was shaken for 2 min using a rotary shaker. Afterward, 4 g of the cold, and (3) establish the relationship between pesticide accumu- NaCl and 6 g of MgSO4 were added to the tubes, and the tubes were lation in sediments and watershed environmental factors and reveal the shaken immediately for 2 min. Then, the samples were centrifuged at impact of long-term land development. 10,000 rpm for 3 min, and 2.0 mL of the acetonitrile layer was passed through Florisil cartridges as clean-up. The cleaned samples were

2. Materials and methods transferred to a 2.0-mL microtube with 0.3 g of MgSO4 to remove the residual water. Finally, 1 mL of extract was evaporated to dryness using 2.1. Study area and sample collection nitrogen stream (Czech et al., 2016). A1-μL sample of sediment extract was analyzed using high-per- To test the hypothesis, channel bed sediment was collected from formance GC–MS (Varian 4000). A silica column (0.25 μm, two river sections within a typical agricultural watershed with mixed 0.25 × 300 mm) was used, and the column conditions were as follows. land use in Northeast China (drainage area of 141.5 km2)(Fig. 1a). This The oven temperature was programmed to range from 50 °C (equili- − area has a continental monsoon climate in a cold temperate zone. The brium time of 1 min) to 180 °C at a rate of 15 °C min 1, and this latter period with an average air temperature below 0˚C lasts approximately temperature was held for 2 min; then, the temperature was decreased − 6 months, and the average content of soil organic matter in the wa- from 280 °C to 190 °C at a rate of 5 °C min 1 (the latter temperature tershed is 6.42% (Ouyang et al., 2012). Since 1992, the watershed was held for 5 min). Helium (99.999%) was used as the carrier gas and − upland area has decreased from 48.65 km2 to 30.44 km2, and the area makeup gas at a constant flow rate of 1.69 mL min 1. The injector of the paddy rice has increased from 0.02 km2 to 49.81 km2 due to the temperature was set at 250 °C. The mass spectrometer was equipped exploitation policy. According to the land use change and the natural with an electron impact ionization (EI) device with an source at environment conditions, two sampling points of the watershed were 230 °C.

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Fig. 1. Location and land use of the watershed and sampling sites of sediment core (a), chemical structures of target pesticides and the major degradation products of atrazine (b)

Table 1 MS parameters and analytical performance of pesticides analyzed by the GC/MS with SIS model.

Pesticide RTa Tb R2,c Rd LODe LOQf

Atrazine 13.775 215.10 > 58.00 0.997 88.1–99.5% 0.10 0.40 Prometryn 16.57 241.20 > 199.10 0.996 87.0–99.4% 1.29 1.60 Isoprothiolane 21.02 290.10 > 118.00 0.996 87.5–97.7% 1.66 1.70 Oxadiazon 21.35 258.00 > 175.00 0.997 90.1–92.8% 0.43 0.60 Deethyl-atrazine 14.535 172.00 > 69.00 0.996 80.1–97.7% 0.15 0.20 Deisopropyl-atrazine 14.720 145.00 > 110.10 0.994 80.8–93.7% 0.10 0.40

a Retention time (min). b Target ion (m/z). c Coefficient of determination. d Recovery (%). e Limit of detection (ng/kg). f Limit of Quantification (ng/kg).

The FULLSCAN model and the Scientific Instrument Services (SIS) in 90.1–92.8%, 80.1–97.7% and 80.8–93.7%, respectively. No further the GC/MS system were both used for the pesticide determinations. A recovery efficiencies were measured, as the recovery percentages of qualitative analysis was conducted using the FULLSCAN model to these pesticides were high in the sediment extraction cases. identify the categories of pesticides (Bonansea et al., 2013). The SIS model was used for quantitative analyses of the concentrations of the 2.3. Sediment core dating and flux calculation six analytes. The retention times and the characteristic of the target analytes are listed in Table 1. To describe the sedimentary process, the vertical distribution of Using the external standards method, the abundances of the char- Lead-210 (210Pb) in each slice was first analyzed. A low-background acteristic ions were determined directly, and standard working curves HPGe (high purity germanium) γ spectrometer was used to measure the ffi 210 226 210 were established based on the acceptable correlation coe cients activity of Pbtot and Ra. The Pbtot activity was determined by 2 (R > 0.994, Table 1). In addition, the standard solutions of the target the 46.5 keV γ-ray and 226Ra activity, which were in equilibrium with 210 analytes were made by diluting the 10 ppm stock solutions which were the Pbsu activity, and were determined by the 95.2 keV and prepared by dissolving 1.00 mg of the standard sample with methanol. 351.9 keV γ-rays. The method LOD and LOQ of the pesticides ranged from 0.1 to 1.66 ng/ Based on the vertical distributions of 210Pb, some geochronology kg and from 0.20 to 1.70 ng/kg, respectively. The recovery rates were models with the 210Pb radioactive decay equation have been developed. determined to evaluate the accuracy and precision of the analytical In this study, the constant sedimentation rate model was employed procedures. The recoveries of each pesticide at five spiked levels varied based on the assumption that the flux of 210Pb was constant in this from 80.1% to 97.7%, with the relative standard deviation below 20%. agricultural watershed (Szmytkiewicz and Zalewska, 2014). The sedi- The recovery efficiencies for atrazine, prometryn, isoprothiolane, ox- mentation rate was calculated using this model by the linear regression – – – 210 adiazon, DEA and DIA, were 88.1 99.5%, 87.0 99.4%, 87.5 97.7%, of Pbex and the depth layer, as follows:

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Fig. 2. Concentrations (upper x-axis) and fluxes (bottom x-axis) of the four pesticides (a. atrazine, b. prometryn, c. isoprothiolane, d. Oxadiazon) in the two sediment cores.

II()x = (0)e−k, the sediment was used to indicate the temporal pattern of the effects of

210 −1 the environmental factors. In this study, six metrics were selected as the where I(0) is the Pb activity at the surface layer (Bq kg d.m.), I(x) is − response variables, including the flux of atrazine, prometryn, iso- the excess 210Pb activity at depth x (Bq kg 1 d.m.), x is the sediment prothiolane, oxadiazon, DEA and DIA in the sediment. The yearly depth (cm), K is the slope defined by regression through the data, and t − precipitation, watershed surface runoff, paddy land area, upland area, is the radioactive decay constant (0.03114 year 1). diffuse total nitrogen and phosphorus load were used as the explanatory With this model, the sediment accumulation rate can be measured variables. The yearly precipitation was derived from the China Me- using the slope of the linear regression of 210Pb, and the vertical sedi- teorological Administration (http://data.cma.cn/data/cdcdetail/ ment pattern can be assessed: dataCode/). The temporal patterns of land use, simulated diffuse total IIm = 0e−bx, nitrogen, phosphorus and surface runoff were referenced from a pre- vious study (Ouyang et al., 2017a). The solid arrows represent the R = tb/ , pesticide flux in the sediment, and the hollow arrows represent the 210 −1 where Im is the excess Pb activity at the cumulative depth x (Bq kg watershed explanatory variables. The cosine of the angle between the d.m.), b is the slope defined by regression through the data, and R is the two rays indicates their correlation. The length of the hollow arrow − − mass sediment accumulation rate (g cm 2 year 1). represents the temporal variation of the indicator, and longer arrows Based on the dated sediment accumulation rate and the pesticide indicate greater relevance. The performance of RDA was evaluated by concentration of each layer, the flux was calculated: eigenvalues, which explained the variation. The eigenvalues of axis1 in S1 and S2 are 0.387 and 0.442, respectively. The explained variation of FCR=××10, RDA shows that the first and second axes of two sections can explain where F is the pesticide accumulation flux at each sediment slice (μg more than 95% of the relationship between the environmental factors − − − m 2 yr 1), C is the pesticide concentration of each slice (ng g 1), and and pesticides. − − R is the sediment mass accumulation rate of each slice (g cm 2 year 1).

2.4. Data analysis 3. Results and discussion

The correlations in the vertical distributions of the pesticide con- 3.1. Vertical distribution of pesticide concentrations and flux in sediments centrations and the fluxes in the same riverine sediment and in sedi- ment from different sections were assessed using correlation coefficient. First, the vertical distributions of the flux and the concentrations of The comparisons of the average pesticide concentration and the flux the four types of pesticides in two sediment cores were compared between watershed sections were determined using the Kolmogorov- (Fig. 2). In the S1 section, the vertical distribution of the pesticides Smirnov Z (K-S) assuming heterogeneity of variance; further, the 5% showed a significant declining trend to the deeper layers. The atrazine − significance level was used (Gomiero et al., 2013). These statistical concentrations of the 15 layers ranged from 2.018 to 3. 678 ng·g 1, and − analyses were conducted using the SPSS 20 (IBM SPSS). the average value was 2.517 ng·g 1. The concentrations of atrazine To quantify the typical pesticide and metabolite pollution dynamics decreased to a depth of 8 cm and then remained relatively stable, − with the driving force differences in the two riverine sediments, the fluctuating by approximately 2 ng·g 1, to the bottom layer. Both pro- temporal patterns of the pesticide flux, precipitation, streamflow and metryn and oxadiazone reached to the deepest of the 17 layers and land-use patterns were imported into redundancy analysis (RDA) shared similar vertical patterns. The concentration of prometryn ranged − − (Angeler et al., 2008). This method permitted the determination of the from 3.223 to 4.522 ng·g 1, and the average value was 3.964 ng·g 1. − relative importance of the long-term diffuse pollution dynamics and The concentration of oxadiazone ranged from 2.562 to 3.025 ng·g 1, − sedimentary processes with respect to land-use changes, and natural with a mean value of 2.833 ng·g 1. Both prometryn and oxadiazone conditions were used as the dependent variables. According to the had obvious peak values at 8 cm, which indicated that they had a si- chronological characteristics of the sediment, the flux at each layer of milar source and similar environmental processes. Isoprothiolane was

4 W. Ouyang, et al. Environment International 139 (2020) 105717 measured only in the top 11 layers, and the concentrations ranged from Table 3 − − 1.496 to 1.866 ng·g 1, with an average value of 1.663 ng·g 1. The Statistical significance of the vertical distributions of four types of pesticides averaged concentration of atrazine in the upper section (3.309 ng/g) between two watershed sections.* was larger than that in the lower section, which coincided with the fact Index Atrazine Prometryn Isoprothiolane Oxadiazon that atrazine was mainly applied in the uplands of the upper stream area (Hively et al., 2011). In contrast, the prometryn, isoprothiolane, Con Flux Con Flux Con Flux Con Flux and oxadiazon, which were mainly used for rice, had much lower K-S 0.003 0.005 0.000 0.000 0.000 0.000 0.016 0.010 concentrations in the upper section sediment than in the lower section R2 0.706 0.574 0.754 0.815 0.528 0.461 0.528 0.881 sediment. The average value of the vertical distribution of prometryn, oxadiazone, and isoprothiolane was 1.857 ng/g, 0.689 ng/g, and * Correlation is significant at the 0.05 level. 2.521 ng/g, respectively. Each pesticide has unique effects based on the crop type; thus, the vertical pesticides concentrations analysis in the correlation between the four types of pesticides was relatively high. The two sediment sections also highlights the impacts of the watershed correlation coefficient value (R2) between the concentrations of ox- land-use distribution (Fairbairn et al., 2015). adiazon and prometryn reached 0.897 (P < 0.05), and the higher R2 The vertical patterns of flux were similar to the concentrations in value between their fluxes indicated that they shared a similar transport the two sections. The flux of the four types of pesticides had greater process. A relatively weak correlation (R = 0.599, P < 0.05) was fluctuations in the upper sediment, S2, than they did in the lower se- found between the concentrations and fluxes of isoprothiolane and diment S1. The flux of isoprothiolane indicated that isoprothiolane was atrazine. These values meant that their sedimentary processes and detected after 1993, which was consistent with the historical change in sources were different. These correlation analyses indicated that atra- the land use of this watershed, which indicates that the result of dating zine and prometryn also shared similar sedimentary processes over past was accurate. In the lower section, atrazine had a clear declining trend decades. The correlations at the lower section S1 presented some dif- over the three-decade sedimentary period, which indicated that the ferences. The closest correlation was between the fluxes of iso- atrazine loss rate in the sediment was stable and there were lower new prothiolane and prometryn, but a weaker correlation still existed be- accumulated loads from farmland (Rodrigues et al., 2013). The mean tween the fluxes of isoprothiolane and atrazine. − − flux of atrazine in the lower section was 12.33 µg·m 2·yr 1, which was In addition, the K-S test was employed to distinguish the differences − − smaller than the flux (16.49 µg·m 2·yr 1) in the upper section. This between the two watershed sections (Table 3). The K-S test of the decrease was the consequence of atrazine degradation and less loading concentrations and fluxes of all four pesticides showed that there were from the downstream watershed (see Table 2). significant differences between the two sections. Based on the differ- However, the distributions of the other three pesticides differed, and ence analysis, the correlation of each pesticide in two sections was their declining slope was not obvious; furthermore, the deposition flux further analyzed. The R2 values of the concentrations and fluxes of showed a peak in 2003, and all had higher values in the sediment core prometryn and oxadiazon were all higher than 0.75 (P < 0.05), which at the watershed outlet. The mean flux of prometryn and isoprothiolane indicated that their vertical distributions were similar. The concentra- − − − − increased intensively, from 9.29 µg·m 2·yr 1 to 19.17 µg·m 2·yr 1 and tion and flux of isoprothiolane had weak relationships with the other − − − 3.47 µg·m 2·yr-1to 7.11 µg·m 2·yr 1, respectively. These variances pesticides and between the different sections, which indicated iso- indicated that most of the loading originated from the paddy land area prothiolane was more sensitive to the sedimentary process, especially in in the downstream region. Under the same situations, the mean ox- the downstream section, which had a higher streamflow value. − − − − adiazone flux increased from 12.56 µg·m 2·yr 1 to 13.52 µg·m 2·yr 1. The R2 value of the atrazine concentration was good, but that of the The fluctuation point in the upper riverine sediment also occurred for flux was poor (Table 3). This difference indicated that atrazine suffered atrazine and oxadiazon in the same year; however, they did not occur several environmental impacts during the sedimentary process. As for prometryn or isoprothiolane. This difference indicated that the po- atrazine was mainly used in the uplands of the upper stream area, the tential loading of atrazine and oxadiazon in the farmland of the upper close correlations between the sections indicated that stable long-term stream area was much larger than that of the other two dues to upland riverine transport was also a key source of the atrazine observed in the tillage. downstream area. In addition to the lower temperature, the higher concentration of organic carbon during the territorial erosion process was another factor that affected the pesticide transport, degradation, 3.2. Deposition and transport characteristics of pesticides in sediments and sedimentary processes in the agricultural watershed. The soil or- ganic carbon (SOC) concentration in the farmland was higher than fl − The correlation of the vertical concentration and ux distributions 23.0–37.4 g kg 1, which contributed to higher dissolved organic of the four types of pesticides were analyzed in the same riverine se- carbon (DOC) in the aquatic system (Ouyang et al., 2014). The DOC is a ff diments and between the di erent sections. The concentration corre- ubiquitous constituent in the aquatic environment, which has a highly lations were usually better than the flux. In the upper stream of S2, the

Table 2 Correlation between pesticide concentrations (Con) and fluxes in sediment cores.*

Upper Section S2 Lower Section S1

Pesticide Atrazine Prometryn Isoprothiolane Oxadiazon Atrazine Prometryn Isoprothiolane Oxadiazon

Con Atrazine 1 0.805 0.599 0.632 1 0.572 0.519 0.822 Prometryn 1 0.701 0.897 1 0.837 0.527 Isoprothiolane 1 0.727 1 0.665 Oxadiazon 1 1

Flux Atrazine 1 0.727 0.437 0.447 1 0.618 0.485 0.737 Prometryn 1 0.657 0.827 1 0.845 0.496 Isoprothiolane 1 0.767 1 0.671 Oxadiazon 1 1

* Correlation is significant at the 0.05 level.

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Fig. 3. Vertical distribution of concentration and flux of atrazine metabolites in sediment cores. specific surface and a strong adsorption capacity for organic atrazine the lower degradation and discharge rate in this cold watershed (Wu compounds (Gao et al., 1998). With the adsorption of DOC, the stronger et al., 2018). It has been shown that the pesticide biodegradation speed riverine transport capability led to the close correlation between the becomes slower under low- conditions in the soil or sediment two sediment sections. (Vargha et al., 2005); thus, due to the lower oxygen concentration in the frozen soil of this cold watershed, the biodegradation speed is much slower. 3.3. Vertical distribution of atrazine metabolites in sediments

The two types of atrazine metabolites were detected in all layers of 3.4. Pesticide metabolite dynamics in the freeze-thaw watershed both river sections, and the lower section had a larger value than the upper section due to the longer degradation period (Fig. 3). At the The vertical distribution of atrazine and its metabolites showed that − lower section S1, the DEA concentration ranged from 18.665 ng·g 1 to the metabolite concentrations exceeded their matrix concentrations − − 19.768 ng·g 1, and the average value was 19.244 ng·g 1. The DIA (Fig. 3). To provide more detailed data to further assess the atrazine − − concentration in S1 ranged from 15.984 ng·g 1 to 18.348 ng·g 1, with degradation process, the concentration ratios of DEA and DIA to atra- − an average value of 17.118 ng·g 1. The corresponding mean values for zine at each layer were also analyzed (Fig. 4). The DEA or DIA/atrazine − DEA and DIA in the upper section were 10.644 and 7.801 ng·g 1, re- ratios were less than 10, which is much smaller than the average value spectively. Their vertical flux patterns were similar to those seen for the of approximately 50 that was determined in sediments collected from − − concentrations. The mean flux of DEA was 94.39 µg·m 2·yr 1 in the South Carolina, United States (Smalling and Aelion, 2006). The differ- − − lower section and 52.76 µg·m 2·yr 1 in the upper section. The DIA flux ences between the two sections were obvious, and the ratio was much − − − − in the two sections was 80.31 µg·m 2·yr 1 and 45.97 µg·m 2·yr 1, larger in the lower stream section. In addition, the ratio in the lower respectively. The concentrations and fluxes of the metabolites were much larger than those observed for atrazine, and their vertical patterns were dif- ferent. DEA had a simple declining pattern in the downstream section, but the peak value of DIA appeared in the middle layers. In the upper stream, DEA had a clear decreasing trend, which supported the tem- poral pattern of atrazine degradation. Additionally, the mutation in the surface might be attributed to the metabolic time of atrazine in the soil, which could not be metabolized completely during the short period. Because atrazine was mainly applied in the upland areas of the upper stream, the higher value of atrazine observed in the lower section in- dicated that the degradation process was intensive throughout the watercourse. In addition, the concentrations of atrazine compounds in the sedi- ment of the watershed have a higher concentration range than the values reported in North Dakota, United States and in Northeast Spain (Lópezflores et al., 2003; Mcmurry et al., 2016). The temperature of this watershed is lower than 0 °C from October to April of the following year, and the range of the freezing-thawing cycle is approximately 15 °C, with only one crop rotation and a regular pesticide application Fig. 4. Vertical distribution of the ratio of atrazine metabolites to atrazine load. The direct reason for the higher concentration in the sediment is concentration in the two sections.

6 W. Ouyang, et al. Environment International 139 (2020) 105717 section had a sharp point at a depth of 7 cm, and the ratios of the two pesticide loading from the paddy land. Because the upland en- types of metabolites were close. Because the vertical pattern also re- compassed the dominant farmland in the upper stream area and the presented the temporal dynamics, it was concluded that the primary transport time was shorter, the RDA distribution of the upper section degradation period for atrazine was approximately 10 years. Afterward, was mainly correlated with a decrease in the upland area, which had an the increased metabolites and the decreased atrazine led to the larger important function. The prometryn (PROM) and isoprothiolane (ISOP) ratio value. The similar ratios of the two metabolites in the lower had close correlations, as did atrazine with the metabolites. The RDA of stream indicated that the degradation speeds were also similar. At the the two sediments indicated that the temporal farmland change was the same depth, there was a slight variation point for the upper stream, but dominant factor that affected pesticide loading in the downstream the difference between the two metabolites was clear. The vertical watershed, and the decreased upland area was the key factor that in- pattern in the upper stream did not vary along the temporal scale, fluenced the upper stream watershed. The poor relationships between which indicated that the degradation speed in the sediment was also the pesticide concentrations and the simulated loads of the two diffuse slow. source nutrient pollutants indicated that their transport and sedimen- Based on the ratio comparison of the two metabolites, it was found tary processes were not the same. that the differences in the two types of metabolites in the same sedi- ment were slight, which indicated that the accumulation and de- gradation processes were mainly related to territorial loading. The ratio 3.6. Implications for pesticide pollution management of a cold agricultural of DEA/ATZ was larger than the ratio of DIA/ATZ, which meant that watershed atrazine was more likely to degrade to DEA (Pionke and Glotfelty, 1990). Additionally, the ratio of the two metabolites increased with The vertical distribution of the pesticides in the sediment is one increased depth, because the water solubility of DEA and DIA is better effective aspect that can be assessed to explore the temporal pattern of than atrazine, which can more easilyer to migrate downward. The pollution loading (Jiao et al., 2015). The vertical distribution of char- comparison between the two sections also demonstrated that the larger acteristics four typical pesticides of the two sections shows the impact ratio gap in the lower section was caused by degradation throughout of long-term changes in land use and the spatial distribution of farm- transport in the watercourse. Because atrazine is mainly released from land on pesticides. Therefore, by combining the vertical pesticide flux the uplands of the upstream area and the relatively stable ratio was and the temporal climate data, the discharge patterns of typical pesti- deeper than 8 cm, it can be concluded that the riverine transport pro- cides from farmland can be estimated. Furthermore, a higher atrazine cess with suspended particles positively influences the degradation concentration and lower ratio with metabolites were found in com- process. parison to other studies, which indicates the slow degradation processes in this watershed. These findings suggest that there should be different guidelines for organic pollution management in cold agricultural wa- 3.5. Driving force difference of the two sections in the same watershed tersheds. The long-term concentration and flux dynamics of pesticides in the The results of the redundancy analysis (RDA) for the four types of sediment was assessed, which provided detailed guidelines for diffuse pesticides and the two types of atrazine metabolites in the sediment pesticide pollution control under the temporal patterns of land-use with the six environmental factors in this watershed is shown in Fig. 5. changes (Zhao et al., 2013). The comparison of the RDA for the two Significant correlations of environmental factors and pesticides were sediments demonstrated the impact of land-use changes on pesticide fi found in the rst axes (RS1 = 0.77, RS2 = 0.74, P < 0.05). In addition, loading, and the impacts of the decreased upland and increased paddy the distributions of the two sections indicated that the land-use varia- rice were both identified (Dagnino et al., 2013). The RDA of the two tion and watercourse transport had direct impacts on the loading and sediment sections also demonstrated the differences in the same wa- degradation of pesticides, and this influence was stronger in the lower tershed, which was mainly a consequence of the land use distributions. section of the entire watershed. The temporal pattern of the decreased upland area had more direct The left figure that depicts the lower stream sediment with the data interactions with the pesticide loadings in the upper stream area. These of the whole watershed showed that the expanding paddy land area was findings indicate that more attention should be focused on preventing close to the loading of prometryn (PROM), isoprothiolane (ISOP) and prometryn and isoprothiolane pollution, especially with the expansion oxadiazon (OXAD), which also coincided with land-use distribution. of paddy rice areas. Next to the paddy land, the precipitation, runoff, and watershed diffuse total nitrogen loading were the other critical factors that affected

Fig. 5. Redundancy analysis of the flux of pesticides and metabolites in sediment with environmental factors.

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4. Conclusions Bhattacharyya, R., Fullen, M.A., Booth, C.A., Kertesz, A., Toth, A., Szalai, Z., 2011. Effectiveness of biological geotextiles for soil and water conservation in different – fi agro-environments. Land Degrad. Rehabil. 22, 495 504. The analysis in this research con rmed that land-use variation and Bonansea, R.I., Amé, M.V., Wunderlin, D.A., 2013. Determination of priority pesticides in agriculture exploitation policy have a large impact on pesticides pol- water samples combining SPE and SPME coupled to GC–MS. A case study: Suquía lution by using the 210Pb dating method and vertical distribution River basin, Argentina). Chemosphere 90, 1860–1869. Boyer, A., Ning, P., Killey, D., Klukas, M., Rowan, D., Simpson, A.J., Passeport, E., 2018. comparisons of the four pesticides in sediments. 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